Symbolism, Imagery, Allegory
The mimoids are formations thrown up by the Solaris ocean, discovered originally by Giese. As Kelvin says, the name that Giese game them "indicates their most astonishing characteristic, the imitation of objects, near or far, external to the ocean itself" (8.55). In other words, the mimoids look like real-world objects; the ocean copies things. The visitors, then, could be seen as a new kind of mimoid, copied from human thoughts.
The mimoids can also be seen as dreams, though, and if Giese was following a dream, that explains perhaps why he "fell in love with the 'mimoids,' and was soon devoting all his time to them" (8.54). If the mimoids are dreams, however, that they are written about suggests that they're the kinds of dreams you write down. In other words, the mimoids could be seen as literature, or literary forms—the great ocean brain trying to write Hamlet, or at least "The Three Little Pigs."
The mimoids, then, are a symbol of writing—and of Solaris itself. Which means that Solaris symbolizes Solaris, and vice versa. (See "What's Up With the Title?" to untangle this mess.) When you read about the mimoids struggling to form images, you can imagine Stanislaw Lem, seated at his desk, brow creased, trying to form the image of a mimoid. And then give him a little whirly cap and a funny false nose with a mustache or anything else that suits your fancy. It's your mimoid, after all, so go with whatever you like.